Thursday, November 15, 2007
Ben Everitt scrambles to un-scramble the slickensides along a major splay of the Hurricane fault
The Canal Trail starts on the South Rim of Virgin River, accessed via Sheep Bridge gravel road south of Virgin Town
November Hiking into Virgin Gorge via Canal Trail
Morning Shadows Create Mystery in Virgin Gorge
Google Earth Map shows Fracture pattern influence on Virgin River
Fissures and Fractures near Virgin River and Town
Access to this Trail may be made by taking the Highway toward Kanab, from Hurricane, and turnng north after reaching the top of the switchbnacks (about 1-2 miles on top of the plateau). This is an improved road going to the town Of Virgin. After 2 miles on this gravel road there will be a trail marker for the Canal Trail. Turn left and proceed on the most used dirt road, until you reach a deadend at the Virgin cliffs.
Alternately, take the Virgin hiway 9 towards Zion NP and turn south across from Mesa Road onto Sheep Bridge road (mostly dirt) for 3 miles (past the flagstone quarry). Look for the Canal Trail marker where you turn right. You can see the Virgin gtorge in the distance, and you will have to feel your way to reach it by seeing an imposing outhouse on the north skylinie.
An interesting (unplanned) experiment occurred near the Hurricane Pipeline in the late 20th century, west of the town of Virgin. This involved the Diversion dam built there to divert water for a pipeline, the Pah Tempe hot springs, fissures in the earth, and The Virgin River flow.
Previously, the 19th century-initiated Hurricane Canal had carried water through an open flume on the south bank to the agricultural area of Hurricane Valley. Because of high maintenance necessary with this canal, a pipe was laid in the canyon of the Virgin valley toward the west, superseding the old canal. There was a large amount of spalling and rock fall annually, so that it was generally known that the cliffs would continue to deteriorate. A covered pipeline would minimize this problem, and the pipe would take water from approximately the same location as the canal- some 5 km west of the town of Virgin and deliver it to farmlands in the valley to the west. This metal pipe would not only eliminate the annual maintenance, but also deliver water more efficiently along the smooth interior of the pipe.
Here is the Third Party-communicated Sequence of Events
A holding reservoir upstream of the pipe was created via a dam across the Virgin, so that water would have time to drop some of its sediment load, and so that there would be a fairly stable flow into the pipeline. The process created an unstable subsurface with the operation of large machinery and with the diversion of water into the pipeline, the Virgin River downstream of the new dam diverted its water into the earth through a sinkhole in the river bed.
Within the first few months, an increased flow of water through Pah Tempe hot springs near the Hurricane fault, Hf, occurred, consisting at first of a slug of hot water. Then the springs delivered colder water (than the original 108 degrees Fahrenheit) making the springs unattractive for public baths. The loss of water near the dam occurred in one area assumed to be a limestone sinkhole, and this was filled with rock debris and isolated with a levee. This eliminated most of the problem of diversion of water into the earth, and allowed an analysis of the plumbing system of the near subsurface (underground water flow).
My analysis of all this (not having been present during the incident) is as follows:
1. Large Fractures were present (seen now in the cliffs and ground surface to the north of the river as fissures and fractures) in the earth below the river. These previously had been partly sealed off by siltation from the muddy river and cementation from precipitation of calcite cement;
Fractures are seen in vertical views as well as horizontally on the surface North of the Virgin (Pine Valley Mountains in the background).
2. Placement of the new dam not only decreased the sediment load in the river water (fluid viscosity decreased), but aggravated the ground surface by action of heavy machinery- both of which increased the movement of water into the earth;
3. The river water moved through solution channels and fractures in the earth downhill towards the Virgin River opening in the Hurricane Cliffs;
Review this Google Map to Notice the difference between Meanders and Fracture-induced river turns (Notice the Sharp Bends, compared to rounded Meanders, seen downstream of Hf- which you may study from a previous Blog entitled "Meanders")
Descent into the Gorge has been simplified by Trail Builders
Some Fractures cross the Virgin River, showing that they are not due to gravity spalling
4. There was a reservoir of hot water available in the caves and solution channels, which was flushed out in advance of the slug of cold river water. This yielded a temporary surge of hot water through Pah Tempe springs;
5. When the cold surge behind the hot water became faster than the upward movement of hot water, the spring temperature decreased; and
6. Finally, after isolating the sinkhole, the hot springs gradually re-heated the rock and the temperature rose to approximately the original equilibrated condition.
Conclusions pertinent to the Nature of the Subsurface near the Virgin River:
A. The Virgin River has a deep canyon (approximately 200+ meters) at this location and this has created spalling cliff walls, falling parallel to the river. But this is not the reason for the large fissures orienting at angles other than those parallel to the river channel in the plateau above; they are due to the similar circumstances for the fissures and sinkholes below the river- they pre-existed the river canyon. The path of the river seems to be determined by the two sets of fractures, seen by the perpendicular sharp bends in the river;
B. These fractures orient in two principal directions- NW-SE and N-S, as measured in the plateau above and in the river paths between bends;
C. That the river has found these (new? < 1 mybp) fractures is demonstrated by the occurrence of river gravels on the plateau above (on the south side, which orient to the SW) before the ancestral Virgin was captured by the present stream flow; and
D. Fractures seen now have been aggravated by spalling of the cliffs, regardless of orientation, but they were not necessarily initiated by the canyon formation.
Enclosed below is a report following a previous hike into the Timpoweap Canyon, describing the replacement of the old canal with a steel pipeline:
April 2007 Field Trip Report, By Ben Everitt, DGS President
The April 28 hiking field trip in Timpoweap Canyon was a great success, even if it was a little quiet out there by myself. The route is well constructed and easily followed, except for one short section that seems to exist only in the mind of the cartographer. Recent geologic mapping by Bob Biek (Hurricane Quadrangle) and Janice Hayden (Virgin Quadrangle) provide good reference to stratigraphy and surface geology. Signage explains the history of the diversions and canals built to lead water out of the canyon to the Hurricane and LaVerkin benches.
The canals are an interesting example of pioneer engineering and sheer determination in the face of difficult topography and geology. The builders made use of breccia zones and open fractures, and the easy tunneling in the gypsiferous units. Irrigators paid dearly for these shortcuts over the next century in lost water and maintenance nightmares. The canals were abandoned in 1985 and replaced with a steel pipeline.
1) Hurricane canal routed through an open fracture in the Fossil Mountain Member
This tour continues the theme that “there is no such thing as bad geology, just different kinds of interesting geology”. The canyon exposes the Timpoweap limestone and Rock Canyon members of the Moenkopi Formation, and the gypsiferous Harrisburg and cliff-forming Fossil Mountain members of the Kaibab Limestone, and the upper gypsiferous Woods Ranch member of the Toroweep Formation. There are excellent exposures of local deformation and breccias associated with paleokarst (ancient collapse due to dissolution of gypsum), and much evidence of continued collapse in recent times. Both the Harrisburg and Woods Ranch members of the Kaibab
formation are tilted and toppling toward the river from both sides, indicating that the thick gypsums in the underlying Toroweep continue to dissolve and undermine the canyon.
2) Just below the new diversion dam, open fractures extend from the top of the Harrisburg Member into the Fossil Mountain Member, but are not yet evident in the pipeline pad.
Above the canyon rim, remnant gravels resting on the lower red member of the Moenkopi mark the course of the Virgin River before there was a Timpoweap Canyon, and therefore before the Hurricane Cliffs were there. Bob Biek (2003, Geologic Map of the Hurricane Quadrangle) estimates the age of this gravel at middle Pleistocene. Since they contain basalt cobbles, they are probably younger than about 1.5 million years. Therefore the entire canyon and its interesting features are geologically quite young.
Test Drilling by the US Bureau of Reclamation in the 1950’s for the proposed Virgin Dam, named for the town of Virgin which it would have flooded, found rock so fractured, cavernous and permeable, that they walked away from it and never looked back. The sink-hole that swallowed the river for 3 months in the spring of 1985 and recharged Pah-Tempe hot spring is described in Everitt and Einert, 1994 (Utah Geological Association Publication 23, p. 189 – 194). These interesting geologic units underlie much of Washington County, and are sure to present challenges as future development moves out into the hinterland.
Tuesday, November 13, 2007
Sullivan's Knoll, SK, west of Hf has basalt base age on order of 200 kybp, cone at 10 kybp
Grass Valley Flow (on Hf, Hurricane fault, south of town 15 km) is slightly less than 1 mybp in age
Cones and Domes are Generally older than 3/10 mybp (.3) East of Hurricane Fault; shown is Crater Hill, near Virgin Town